topic drift

topic drift /n./ Term used on GEnie, Usenet and other
electronic fora to describe the tendency of a thread to drift
away from the original subject of discussion (and thus, from the
Subject header of the originating message), or the results of that
tendency. Often used in gentle reminders that the discussion has
strayed off any useful track. "I think we started with a question
about Niven's last book, but we've ended up discussing the sexual
habits of the common marmoset. Now *that's* topic drift!"